The subject matter disclosed herein relates to X-ray imaging and, in particular, to the improvement of the effective dynamic range achieved by a detector used in an imaging process.
Non-invasive imaging technologies allow images of the internal structures or features of a patient or object to be obtained without performing an invasive procedure on the patient or object. In particular, such non-invasive imaging technologies rely on various physical principles, such as the differential transmission of X-rays through the target volume or the reflection of acoustic waves, to acquire data and to construct images or otherwise represent the observed internal features of the patient or object.
For example, in computed tomography (CT) and other X-ray based imaging technologies, X-ray radiation is directed toward a subject, typically a patient in a medical diagnostic application, a package or baggage in a security screening application, or a fabricated component in an industrial quality control or inspection application. A portion of the radiation impacts a detector where the image data is collected. In digital X-ray systems, a detector generates digital signals representative of the amount or intensity of radiation impacting discrete pixel regions of the detector surface. The signals may then be processed to generate an image that may be displayed for review. In the images produced by such systems, it may be possible to identify and examine the internal structures and organs within a patient's body, objects within a package or container, or defects (e.g., cracks) within a fabricated component. In volumetric imaging systems (such as computed tomography (CT), tomosynthesis, or C-arm angiography systems) a detector array, including a series of detector elements, produces similar signals through various positions as one or both of the source and detector are displaced around the imaged volume, allowing data to be acquired over a limited or complete angular range.
X-ray detection at the detector often utilizes a scintillator, which converts higher-energy X-ray radiation to lower-energy light photons that are sensed using photo-sensitive components (e.g., photodiodes or other suitable photodetectors). The detector is typically divided into a matrix of discrete picture elements or pixels, and encodes output signals based upon the quantity or intensity of the radiation impacting each pixel region. The signals may then be processed to generate an image that may be displayed for review.
In certain instances, during an X-ray exposure where an object is being imaged but relatively little attenuation is present (e.g., due to the size, positioning, or structure of the imaged object, such as at a skin line or tissue boundary) there is a possibility that portions of the detector array in this region of insufficient attenuation will be saturated, while other areas where more attenuation is present are not. That is, the accumulated charge at a given pixel or set of pixels may reach a limit, such that additional exposure does not result in a corresponding increase in measured charge at the pixel. Saturation leads to the loss of information and is characteristic of a detector having insufficient dynamic range to accommodate the X-ray exposure levels, in this instance a failure to accommodate the highest levels of radiation observed at imaged regions of interest, such as near the tissue edge or skin line.
Conversely, under certain short exposure conditions (e.g., in a tomosynthesis frame in three-dimensional (3D) mammography) the electronic noise of the detector, which may be attributable to the readout process, can become significant compared to the inherent X-ray quantum noise, thus impacting the final image quality due to the inability to distinguish small measured signals (relative to the electronic noise). Such a circumstance may be characteristic of a detector having insufficient dynamic range to accommodate the X-ray exposure levels, though in this instance the failure is in an inability to accommodate the lowest levels of radiation in view of the electronic noise that may be present.